Method to improve reliability of replacement gate device

ABSTRACT

A method of fabricating a replacement gate stack for a semiconductor device includes the following steps after removal of a dummy gate: growing a high-k dielectric layer over the area vacated by the dummy gate; depositing a thin metal layer over the high-k dielectric layer; depositing a sacrificial layer over the thin metal layer; performing a first rapid thermal anneal; removing the sacrificial layer; and depositing a metal layer of low resistivity metal for gap fill.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of, and claims priority to copending U.S.patent application Ser. No. 14/595,756, filed on Jan. 13, 2015, whichwas in turn a division of issued U.S. Pat. No. 8,999,831 issued on Apr.7, 2015, both incorporated by reference in their entirety, and whereinsuch applications were made by, on behalf of, and/or in connection withthe following parties to a joint research agreement: InternationalBusiness Machines Corporation and GlobalFoundries. The agreement was ineffect on and before the date the claimed invention was made, and theclaimed invention was made as a result of activities undertaken withinthe scope of the agreement.

FIELD OF THE INVENTION

The invention disclosed broadly relates to the field of integratedcircuit fabrication, and more particularly relates to improving thereliability of high-k transistors using a gate-last fabrication process.

BACKGROUND OF THE INVENTION

In the semiconductor industry, Moore's law states that the number oftransistors on a chip doubles approximately every two years. Theseexponential performance gains present a challenge to the semiconductormanufacturing industry, along with the dual challenges of promotingpower savings and providing cooling efficiency. The industry addressesthese challenges in multiple ways. Selecting the gate dielectric andgate electrode are critical choices in enabling device scaling, andcompatibility with CMOS technology. Two main approaches have emerged inhigh-k and metal gate (HKMG) integration: gate-first and gate-last.Gate-last is also called replacement metal gate (RMG) where the gateelectrode is deposited after S/D junctions are formed and the high-kgate dielectric is deposited at the beginning of the process (high-kfirst).

A high-k first gate-last process is when the high-k dielectric isdeposited first and the metal is deposited last (gate-last method).Gate-last is often referred to as the replacement gate option. “First”and “last”—gate denotes whether the metal gate electrode is depositedbefore or after the high temperature anneal process. Typically, thereliability of high-k gate stacks improve as a result of dopantactivation anneal at a temperature of about 1000° C. However, thisannealing process is only used for gate-first or high-k first, metalgate-last processes. The high-k last, metal gate-last process lacks suchbuilt-in high temperature treatment and thus reliability is a bigchallenge.

In the conventional process, if we want to apply a high thermal budgeton high-k metals to improve reliability, the high-k metal layer needs tobe formed prior to the dopant activation anneal (this is so-calledgate-first process). The gate-first process typically requires robustencapsulation (using spacers) of the high-k metal gate stacks to preventambient oxygen to affect device characteristics. In addition, the high-kmetal gate stack needs to be etched by RIE (reactive ion etching) at thetime of gate patterning, which is typically challenging.

We provide a glossary of terms used throughout this disclosure:

GLOSSARY

k—dielectric constant value

high-k—having a ‘k’ value higher than 3.9 k, the dielectric constant ofsilicon dioxide

RTA—rapid thermal anneal.

A-Si—amorphous silicon

ALD—atomic layer deposition

CMOS—complementary metal-oxide semiconductor

FET—field effect transistor

FinFET—a fin-based, multigate FET

MOSFET—a metal-oxide semiconductor FET

PVD—physical vapor deposition

SiOx—silicon oxide

SiGe—silicon germanide

SiC—silicon carbide

RIE—reactive ion etching

ODL—optically dense layer; organically dielectric layer

STI—shallow trench isolation

S/D—source and drain terminals

NiSi—nickel silicide

C (DLC)—metal-free diamond-like carbon coating

SiN—silicon nitride

TDDB—time dependent dielectric breakdown

NBTI—negative bias temperature instability

PBTI—positive bias temperature instability

RTA—rapid thermal annealing

IL/HK—interfacial layer/high-k dielectric layer

TiN—titanium nitride

TiC—titanium carbide

TaN—tantalum nitride

TaC—tantalum carbide

TiAl—titanium aluminide

N2—nitrogen

Al—aluminide

W—tungsten

SUMMARY OF THE INVENTION

Briefly, according to an embodiment of the invention a method offabricating a gate stack for a semiconductor device includes thefollowing steps after removal of a dummy gate: growing a high-kdielectric layer over the area vacated by the dummy gate; depositing athin metal layer over the high-k dielectric layer; depositing asacrificial layer over the thin metal layer; annealing the structure ata high temperature of not less than 800° C.; removing the sacrificiallayer; and depositing a metal layer of low resistivity metal for gapfill. Optionally, a second annealing step can be performed after thefirst anneal. This second anneal is performed as a millisecond annealusing a flash lamp or a laser.

According to another embodiment of the present invention, a method offabricating a gate stack for a semiconductor device includes thefollowing steps after removal of a dummy gate: growing a high-kdielectric layer over an area vacated by the dummy gate; depositing athin metal layer over the high-k dielectric layer; depositing asacrificial layer over the thin metal layer; annealing the replacementgate structure at a high temperature of not less than 800° C.; removingboth the thin metal layer and the sacrificial layer; performing a secondrapid thermal anneal, this time at a temperature range between 400° C.and 800° C., inclusive; re-depositing a thin metal layer over the high-kdielectric layer; and depositing a metal layer for gap fill.

According to another embodiment of the present invention, a method offabricating a gate stack for a semiconductor device includes thefollowing steps after removal of a dummy gate: growing a high-kdielectric layer over an area vacated by the dummy gate; depositing athin metal layer over the high-k dielectric layer; depositing asacrificial layer over the thin metal layer; annealing the replacementgate structure at a high temperature of not less than 800° C.;performing a millisecond anneal; removing both the thin metal layer andthe sacrificial layer; performing a second rapid thermal anneal, thistime at a temperature range between 400° C. and 800° C., inclusive;re-depositing a thin metal layer over the high-k dielectric layer; anddepositing a metal layer for gap fill.

According to another embodiment of the present invention, a method offabricating a gate stack for a FinFET device includes the followingsteps after removal of a dummy gate: growing a high-k dielectric layerover the area vacated by the dummy gate; depositing a thin metal layerover the high-k dielectric layer; depositing a sacrificial layer overthe thin metal layer; annealing the structure at a high temperature ofnot less than 800° C.; removing the sacrificial layer; and depositing ametal layer of low resistivity metal for gap fill. Optionally, a secondannealing step can be performed after the first anneal. This secondanneal is performed as a millisecond anneal using a flash lamp or alaser.

According to another embodiment of the present invention, a method offabricating a gate stack for a FinFET device includes the followingsteps after removal of a dummy gate: growing a high-k dielectric layerover an area vacated by the dummy gate; depositing a thin metal layerover the high-k dielectric layer; depositing a sacrificial layer overthe thin metal layer; annealing the replacement gate structure at a hightemperature of not less than 800° C.; performing a millisecond anneal;removing both the thin metal layer and the sacrificial layer; performinga second rapid thermal anneal, this time at a temperature range between400° C. and 800° C., inclusive; re-depositing a thin metal layer overthe high-k dielectric layer; and depositing a metal layer for gap fill.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

To describe the foregoing and other exemplary purposes, aspects, andadvantages, we use the following detailed description of an exemplaryembodiment of the invention with reference to the drawings, in which:

FIGS. 1A through 1D illustrate a replacement gate formation process,according to an embodiment of the present invention;

FIG. 1A is a simplified illustration of a gate structure after removalof a dummy gate, according to an embodiment of the present invention;

FIG. 1B is a simplified illustration of the gate structure of FIG. 1Aafter deposition of a gate metal layer and a sacrificial Si layer,followed by a RTA, according to an embodiment of the present invention;

FIG. 1C is a simplified illustration of the gate structure of FIG. 1Bafter removal of the sacrificial Si layer, according to an embodiment ofthe present invention;

FIG. 1D is a simplified illustration of the gate structure of FIG. 1Cafter deposition of a work function metal and gap fill metal, accordingto an embodiment of the present invention;

FIGS. 2A through 2F illustrate a replacement gate formation process,according to another embodiment of the present invention;

FIG. 2A is a simplified illustration of a gate structure after removalof a dummy gate, according to an embodiment of the present invention;

FIG. 2B is a simplified illustration of the gate structure of FIG. 2Aafter deposition of a gate metal layer and a sacrificial Si layer,following by a RTA, according to an embodiment of the present invention;

FIG. 2C is a simplified illustration of the gate structure of FIG. 2Bafter removal of the sacrificial Si layer, according to an embodiment ofthe present invention;

FIG. 2D is a simplified illustration of the gate structure of FIG. 2C,after removal of the thin metal layer, followed by an optional RTA,according to an embodiment of the present invention;

FIG. 2E is a simplified illustration of the gate structure of FIG. 2D,after deposition of the thin metal layer previously removed, accordingto an embodiment of the present invention;

FIG. 2F is a simplified illustration of the gate structure of FIG. 2Eafter deposition of work function and fill metals, according to anembodiment of the present invention;

FIG. 3 is a flowchart of the method of forming the replacement gateshown in FIGS. 1A through 1D, according to an embodiment of the presentinvention; and

FIG. 4 is a flowchart of the method of forming the replacement gateshown in FIGS. 2A through 2F, according to an embodiment of the presentinvention.

While the invention as claimed can be modified into alternative forms,specific embodiments thereof are shown by way of example in the drawingsand will herein be described in detail. It should be understood,however, that the drawings and detailed description thereto are notintended to limit the invention to the particular form disclosed, but onthe contrary, the intention is to cover all modifications, equivalentsand alternatives falling within the scope of the present invention.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with thepresent invention, it should be observed that the embodiments have beenrepresented where appropriate by conventional symbols in the drawings,showing only those specific details that are pertinent to understandingthe embodiments of the present invention so as not to obscure thedisclosure with details that will be readily apparent to those ofordinary skill in the art having the benefit of the description herein.Thus, it will be appreciated that for simplicity and clarity ofillustration, common and well-understood elements that are useful ornecessary in a commercially feasible embodiment may not be depicted inorder to facilitate a less obstructed view of these various embodiments.

We describe a gate-last, high-k metal gate with a novel improvement inreliability. We enable a high thermal budget treatment on high-k metalgate stacks while avoiding the aforementioned challenges of requiringetching at the time of gate patterning, and requiring a robustencapsulation of the high-k metal gate stack. We achieve our reliabilityimprovement by adding a sacrificial layer and a high temperature annealstep to the high-k, gate-last formation process. The sacrificial layeris a silicon (Si) layer that we deposit after removing the dummy gatestructure. By employing the sacrificial Si layer, followed by a hightemperature anneal (800 to 1100° C.), we thus improve the devicereliability. The sacrificial Si layer allows the temperature increasefor the anneal process.

We further deviate from known methods in that our replacement gateprocess is performed without a silicide contact on the gate.Additionally, the high temperature anneal step in this process can beoptionally used for the dopant activation traditionally used at the timeof the source/drain junction formation. Then the annealing step usuallyperformed at the source/drain junction formation can be skipped.

Referring now in specific detail to the drawings and to FIGS. 1A through1D in particular, we show simplified illustrations depicting thereplacement gate process, according to one embodiment of the presentinvention. This embodiment can be advantageously implemented in variousCMOS devices, including FinFET devices. In this embodiment, we allow forone additional optional anneal. In FIG. 1A we show the gate structure100 after removal of the dummy (sacrificial) gate. We grow aninterfacial layer and deposit a high-k dielectric 110.

In FIG. 1B, we deposit a gate metal layer 120, followed by deposition ofa sacrificial amorphous or poly-crystalline Si layer 130. The gate metallayer 120 in this embodiment is a thin metal layer with a thickness ofapproximately 10 to 50 angstroms. It is preferably a thermally stablemetal alloy, such as TiN, TiC, TaN, or TaC. The gate metal layer 120 canbe deposited via atomic layer deposition (ALD) or physical vapordeposition (PVD). After deposition of the thin metal layer 120, and thesacrificial Si layer 130, we follow with a rapid (spike to 5 seconds)thermal anneal at high temperatures ranging from 800° C. to 1100° C.Spike is a type of RTA where temperatures ramp up and down quickly andthe duration at the maximum temperature is almost zero. In oneembodiment the annealing is performed in ambient nitrogen. After theRTA, we can follow with an optional millisecond anneal, using perhaps alaser anneal or a flash lamp anneal. This optional anneal is carried outfor a very short amount of time. Without limiting the process window, weperform this anneal within a range of 1 to 100 milliseconds.

In FIG. 1C we remove the sacrificial Si layer 130, leaving the thinmetal layer 120 on the gate structure 100. FIG. 1D we deposit a workfunction metal and gap fill metal 140 to finish the replacement gate100. The work function metal 140 can be a metal alloy, such as TiAl orTiN. It serves the purpose of setting the threshold voltage of thedevice to appropriate values. The gap fill metal 140 can be Al, or W.

The benefits and advantages in using this fabrication process for agate-last high-k metal gate are:

-   -   1. High thermal budget in full replacement gate process.    -   2. Reliability (PBTI, NBTI, TDDB) improvement;    -   3. Simplified gate formation process (RIE, encapsulation), which        enables closer proximity of stress elements to gate.

Referring now to FIGS. 2A through 2F, we present simplified diagrams ofthe replacement gate formation process, according to another embodimentof the present invention. This embodiment can also be advantageouslyimplemented in various CMOS devices, including FinFETs. In thisembodiment, we allow for two optional annealing processes. FIGS. 2Athrough 2C are the same steps as in the previous FIGS. 1A through 1C. InFIG. 2A we grow an interfacial layer and deposit a high-k dielectric 110after removal of the dummy (sacrificial) gate. In FIG. 2B, we deposit agate metal layer 120, followed by deposition of a sacrificial amorphousor poly-crystalline Si layer 130. The gate metal layer 120 in thisembodiment, just as in the previous embodiment, is a thin metal layerwith a thickness of approximately 10 to 50 angstroms. It is preferably athermally stable metal alloy, such as TiN, TiC, TaN, or TaC. The gatemetal layer 120 can be deposited via atomic layer deposition (ALD) orphysical vapor deposition (PVD).

After deposition of the thin metal layer 120 and the sacrificial Silayer 130, we follow with a rapid thermal anneal 140 at hightemperatures ranging from 800° C. to 1100° C. After the RTA 140, we canfollow with an optional millisecond anneal 148, using perhaps a laseranneal or a flash lamp anneal. In FIG. 2C we remove the sacrificial Silayer 130, leaving the thin metal layer 120.

In FIG. 2D we remove the thin metal layer 120 in a wet removal process,immediately followed by an optional second RTA 145 at 400° C.-800° C.for 30 seconds in N2 (ambient nitrogen). In FIG. 2E we re-deposit thethin metal layer 120. In one embodiment where we do not perform theoptional second RTA 145, we do not need to remove and consequentlyre-deposit the thin metal layer 120. Lastly, in FIG. 2F we deposit thework function and fill metals 150. This last step correlates to FIG. 1Dof the previous embodiment.

FinFET embodiment.

FinFET is commonly used to describe any fin-based, multigate transistorarchitecture regardless of number of gates. The same process as in theprevious embodiment for a planar structure can be applied to a FinFETstructure, except that high-k and metal films need to be deposited in aconformal manner to obtain desired device characteristics on the 3-D finstructure. This requirement limits the deposition for the high-kdielectric 110, the gate metal layer 120, and the work function metal140 to conformal methods, such as atomic layer deposition (ALD).

We will now discuss the process steps for gate last high-k gatefabrication with respect to the flowcharts of FIGS. 3 and 4. Optionalsteps are depicted in dotted boxes. It will be apparent to those withknowledge in the art that the fabrication of a gate stack on asemiconductor device involves more steps than are shown in FIGS. 3 and4. For example, we skip over the source/drain junction formation andshow the process after the dummy gate has been removed. For clarity, weconcentrate our explanation on those steps that deviate from theconventional fabrication of the high-k gate.

Referring now to FIG. 3, we show a flowchart 300 of the process forfabricating a gate-last high-k metal gate 100 according to theembodiment of FIGS. 1A through 1D. In step 310 we grow an interfaciallayer and deposit a high-k metal 110 after the dummy gate removal. Instep 320 we deposit the gate metal layer 120 and the sacrificial Silayer 130. This is followed by a RTA 140 of 800° C. to 1100° C. in step330.

Next, we can have a second, optional millisecond anneal 148 in step 340.After the annealing process, we remove the sacrificial silicon layer 130in step 350. Lastly, we deposit a metal layer 150 consisting of a workfunction setting metal and a gap fill metal 150 of low resistivity. Thebenefits and advantages to this embodiment are:

-   -   1. Reliability improvement; and    -   2. Simplification of the gate formation process (RIE,        encapsulation), which enables closer proximity of stress        elements to gate.

Referring now to FIG. 4, we show a flowchart 400 of the process forfabricating a gate-last high-k metal gate 200 according to theembodiment of FIGS. 2A through 2F. In step 410 we perform the RTA 140after deposition of the gate metal 120 and Si layers 130. Note that thereason for applying the sacrificial Si layer 130 is to allow theannealing at higher temperatures than would normally be advised. Oncethe high temperature annealing process is complete, the Si layer 130 canbe removed. In optional step 420 we can perform a millisecond anneal148. We use very high temperatures ranging from 1100° C. to 1300° C. forthe millisecond anneal.

In step 430 we remove the sacrificial Si layer 130. Then we remove thegate metal (thin metal layer 120) in step 440. In optional step 450 wecan perform a second RTA 145 with temperatures between 400° C. and 800°C. Note that in this case we were able to perform a RTA 145 afterremoving the Si layer 130 because we did not use such high temperatures.Lastly, we finish the replacement gate in step 460 by depositing thework function and gap fill metals 150 for gap fill using low resistivitymetals. The benefits and advantages to the embodiment of FIG. 4 are:

-   -   1. lower defect density owing to lift-off effect of Si residue    -   2. improved manufacturability    -   3. further recovery of oxygen vacancies in high-k layer by        replacing the sacrificial thin metal layer which leads to        improved gate leakage/reliability.

Benefits 1 and 2 are due to the removal of the thin metal layer 120 andbenefit 3 is due to the combination of removal of the thin metal layer120 and optional second RTA 145.

Therefore, while there has been described what is presently consideredto be the preferred embodiment, it will understood by those skilled inthe art that other modifications can be made within the spirit of theinvention. The above description(s) of embodiment(s) is not intended tobe exhaustive or limiting in scope. The embodiment(s), as described,were chosen in order to explain the principles of the invention, showits practical application, and enable those with ordinary skill in theart to understand how to make and use the invention. It should beunderstood that the invention is not limited to the embodiment(s)described above, but rather should be interpreted within the fullmeaning and scope of the appended claims.

The invention claimed is:
 1. A method of fabricating a gate stack for aFinFET semiconductor device, said method comprising steps of: afterremoval of a dummy gate, providing a replacement gate structure byperforming steps of: growing a high-k dielectric layer over thereplacement gate structure; depositing a thin metal layer over thehigh-k dielectric layer; depositing a sacrificial layer over the thinmetal layer; performing a first rapid thermal anneal of the high-kdielectric layer, the thin metal layer, and the sacrificial layer at ahigh temperature not less than 800° C.; performing a millisecond anneal;removing the sacrificial layer; removing the thin metal layer;performing a second rapid thermal anneal at a temperature between 400°C. and 800° C., inclusive; re-depositing a thin metal layer over thehigh-k dielectric layer and depositing a metal layer for gap fill. 2.The method of claim 1, wherein performing the first rapid thermal annealcomprises performing a rapid thermal anneal at a temperature between800° C. and 1100° C., inclusive, in ambient nitrogen.